Flue gases sulfur dioxide removal

While the use of low-sulfur fuels is one mechanism to reduce sulfur dioxide emission, alternatively most approaches focus on scrubbing or ridding the emissions in smoke stacks of sulfur dioxide gas. A number of different types of scrubbers, i.e., sulfur dioxide removal systems, are available for industry. One system sprays the flue gas into a liquid solution of sodium hydroxide. The hydroxide combines with SO2 and O2 to form the corresponding sulfate which can be removed from the aqueous solution ... 

Sulfur dioxide removal processes can be used to treat flue gas from industrial boilers, heaters, or other process gases where sulfur compounds are oxidized. These processes have generally been proven in utility applications. More recently, several industrial SO2 removal installations have been completed. 

Feeding solutions from the absorber system and the regeneration system through surge tanks enables the entire recovery process to operate smoothly and reliably despite frequent gas flow and concentration fluctuations. In addition, the surge tanks allow the regeneration section to be shut down for up to 3 days without interfering with the sulfur dioxide removal in the absorption section. This is possible because the absorber is the only part of the system that contacts the flue gas and removes the sulfur dioxide. 

The first pilot scrubber tests were conducted using simulated flue gas to establish the feasibility of sulfur dioxide s reacting with sodium carbonate solutions and slurries in a spray dryer. Subsequent tests were conducted at the Mohave generating station, where a 5-ft diameter modified spray dryer was used to test sulfur dioxide removal from a side stream of flue gas from this coal-fired power plant (Figure 4). The spray dryer had been in operation for over 20 yr in various drying applications prior to modification to a sulfur dioxide scrubber. It was used in over 100 tests at Mohave without a single operational problem. 

Gas cooling, cleaning, and sulfur dioxide removal is accomplished by adiabatically cooling flue gas with quench water, passing into a venturi-type water scrubber to remove fly ash, followed by absorption of the sulfur dioxide in an aqueous solution of sodium citrate and citric acid. The pilot plant has demonstrated the feasibility of a commercial plant consistently to remove more than 95% of the sulfur dioxide in the inlet gas. The pilot unit has operated for prolonged periods with exit gas of 25-50 ppm sulfur dioxide. 

It is the most common FGD process—the flue gas is treated with limestone slurry for the sulfur dioxide removal. Approximately 95% of the SOj from the flue gas can be eliminated. The method can be used for medium-to high-sulfur coals. A slurry waste or a saleable slurry by-product is obtained. 

Hydrated lime is used to remove the sulfur dioxide, sulfur trioxide, and hydrogen chloride from the flue gas in this method. Water is injected into the bed to obtain an operation close to the adiabatic saturation temperature. More than 95% sulfur dioxide removal efficiency can be achieved by using this process. The final product is a dry powdered mixture of calcium compounds requiring disposal operations. 

Sulfur dioxide removal experiments were performed in a laboratory scale spray drier (Lab-Plant SD-04). The air flow rate was kept constant at 16.3 m /h during all experiments. The SOj content of the gas was adjusted to 0.2%. Simulated flue gas was prepared by mixing pure SO2 with air. Relative humidity in the spray drier was adjusted by changing the flow rate and composition of the liquid feed rate. The lime slurry was prepared by mixing the mixture on a hot plate at 90°C for about an hour. A schematic diagram of the unit is given in Figure 1. The... 

The control of both SO2 and NO, concentrations in flue gas by a single process has obvious advantages over the use of two separate processes, and considerable effoit has gone into attempts to develop an economical combined NO,/SO, control process. Unfortunately, no combined process has yet achieved widespread acceptance, although numerous approaches have been proposed and a few have reached commercial status. Combined NO,/SO, control processes that have been used commercially, or are believed to be in an advanced stage of development, are described in Chapter 7, Sulfur Dioxide Removal. To avoid repetition, this section is limited to comparative evaluations of available combined processes and very brief descriptions of selected technologies. 

Catalysts may therefore be designed for nse in specific duties. For power plant, the design must balance the reaction rates of NOX reduction and sulfur dioxide oxidation in the restricted range of temperature of flue gas leaving the boiler, or at the dust and sulfur dioxide removal stages. A low activity catalyst that reaches maximum NOX reduction between, say 380°-400°C, can be more efficient than a catalyst that is more active between 300°-350°C because, overall, it produces less sulfur trioxide at the fixed operating temperature. ... 

In magnesium casting, sulfur dioxide is employed as an inert blanketing gas. Another foundry appHcation is as a rapid curing catalyst for furfuryl resins in cores. Surprisingly, in view of the many efforts to remove sulfur dioxide from flue gases, there are situations where sulfur dioxide is deHberately introduced. In power plants burning low sulfur coal and where particulate stack emissions are a problem, a controUed amount of sulfur dioxide injection improves particulate removal. 

Minor and potential new uses for ammonium thiosulfate include flue-gas desulfurization (76,77), removal of nitrogen oxides and sulfur dioxide from flue gases (78,79), converting sulfur ia hydrocarbons to a water-soluble form (80), and converting cellulose to hydrocarbons (81,82) (see Sulfur REMOVAL AND RECOVERY). 

Today s major emissions control methods are sorbent injection and flue gas desulfurization. Sorbent injection involves adding an alkali compound to the coal combustion gases for reaction with the sulfur dioxide. Typical calcium sorbents include lime and variants of lime. Sodium-based compounds are also used. Sorbent injection processes remove 30 to 60% of sulfur oxide emissions. 

Future legislation will stimulate burner development in the areas of carbon monoxide, NOx and particulate generation. Techniques will include flue-gas recirculation, staged combustion, and additives to reduce the NOx and more sophisticated controls. Controls over the sulfur generated do not affect burner design greatly since the sulfur dioxide is a natural product of combustion and can only be reduced by lower fuel sulfur contents or sulfur removal from the exhaust gases. 

The 1970 s also brought about increased use of three-phase systems in environmental applications. A three-phase fluidized bed system, known as the Turbulent Bed Contactor, was commercially used in the 1970 s to remove sulfur dioxide and particulates from flue gas generated by coal combustion processes. This wet scrubbing process experienced several... 

Citrate A process for flue-gas desulfurization by absorption of the sulfur dioxide in aqueous sodium citrate, reacting with hydrogen sulfide to produce elemental sulfur, and recycling the citrate solution. Subsequent modifications involved removing the sulfur dioxide from the citrate solution by steam stripping, or by vacuuming the gas used to make sulfuric acid. 

Flakt-Boliden A variation on the Citrate process for flue-gas desulfurization in which the sulfur dioxide is removed from the citrate solution by vacuum. Developed by Flakt, United States, and piloted in 1980 at the TVA Electric Power Research Institute, Muscle Shoals, AL. 

Kranz MWS A flue-gas desulfurization system based on activated carbon. One carbon bed removes most of the sulfur dioxide. Ammonia is then injected for the SCR process to occur in the second bed, which also removes the residual sulfur dioxide. The carbon is regenerated off-site. Developed by Krantz Company, Germany. In 1986, three plants were operating in Germany. 

MercOx A process for removing mercury and sulfur dioxide from flue-gases. Hydrogen peroxide is first sprayed into the gas, converting metallic mercury to mercuric ions in solution. A water spray removes the sulfur dioxide as sulfuric acid. Mercury is removed from the liquor by ion-exchange, and the sulphate is precipitated as gypsum. Developed by Uhde and Gotaverken, with the Institut fur Technische Chemie. 

Stackpol 150 Also known as IFP Stackpol 150. A flue-gas desulfurization process. The sulfur dioxide is removed by scrubbing with aqueous ammonia, and the sulfur is then recovered by a four-stage process. Developed by the Institut Frangais du Petrole. 

SULF-X [Sulfur extraction] A regenerable flue-gas desulfurization process in which the sulfur dioxide is absorbed by aqueous sodium sulfide in a bed packed with pyrite. Ferrous sulfate is produced this is removed by centrifugation and calcined with coke and fresh pyrite. Sulfur vapor is evolved and condensed, and the residue is re-used in the scrubber. Piloted in the mid-1980s. Not to be confused with Sulfex or Sulph-X. 

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